Ink jet printing is a non-impact printing process in which droplets of ink are deposited on a substrate such as paper or transparency film in response to an electronic signal. Low cost and high quality of the output, combined with relatively noise free operation, have made ink jet printers a popular option to other types of printers used with computers.
Dyes and pigments are used as colorants in inks adapted for use in ink jet printers. Dyes typically provide superior color properties immediately after printing, but are light sensitive and the printed image will tend to fade. Dyes also remain water-soluble after printing, causing the printed image to smear when contacted with moisture.
Pigment colorants offer superior light-fastness and water-fastness, compared to dyes. Polymeric dispersants are employed to form a stable dispersion of the pigment particles in the ink. Representative dispersants proposed for this purpose include polyvinyl alcohol, cellulosic materials, ethylene oxide modified phenols, and ethylene oxide/propylene oxide polymers. More recently, commercial inks have employed a structured polymer dispersant as disclosed in U.S. Pat. No. 5,085,698 to Ma et al. While inks containing pigment colorants generally offer superior water-fastness (compared to inks containing dyes), it is desirable that further improvements be made, particularly for end-use applications wherein the printed image will be exposed to substantial moisture. Water soluble polymeric binders and emulsions have been proposed as additives for this purpose, as in U.S. Pat. No. 5,085,698 to Ma et al. When added at concentrations high enough to form tough protective films, however, it has been found that such additives generally increase the ink viscosity to the extent that the ink will not print in conventional ink jet printers. Accordingly, there is a need for specific additives that will improve water-fastness of inks containing pigment dispersions that are particularly adapted for use in ink jet printers.